A major obstacle to developing an effective HIV-1 vaccine is in determining how to elicit broadly neutralizing antibodies (NtAb). It has been shown in passive immunization studies in animal model systems that neutralizing antibodies can block infection by HIV-1 (or SHIV). Thus, findings ways to elicit potent NtAb responses that can block circulating HIV-1 strains is a high priority for the vaccine field. However, the repertoire of broadly neutralizing antibodies under study, particularly antibodies from individuals infected with the more prevalent subtypes worldwide (A, C, D), is very limited. In addition, these antibodies are not particularly effective at neutralizing the envelope protein of transmitted strains of HIV-1, which have distinct characteristics compared to those present at later stages in infection. Here, we propose to use natural history studies in HIV-1 infected humans to identify examples of individuals who develop broad and potent neutralizing antibodies to their HIV-1 infection. We developed a longitudinal cohort study of women infected with subtypes A, C and D in Mombasa, Kenya in 1993. Longitudinal samples have been collected and stored from this cohort during that time, and we plan to continue to monitor these women. We propose to use the power of this unique longitudinal study to address the hypothesis that there are individuals who have broadly neutralizing antibodies capable of blocking transmitted strains. Here we propose to identify such individuals, and to isolate the corresponding antibodies. The Aims of this grant are: 1) To identify individuals from a group of ~120 women with >5 years of follow-up after their infection who have broad NtAb responses, and specifically have broad and potent NtAbs against recently transmitted strains of subtypes A, C and D. 2) To develop a competition assay that will distinguish cases where breadth is due to one or a few antibodies with broad specificity versus those where the breadth simply reflects a compilation of many antibodies of narrow specificity. 3) To isolate antibodies from subject(s) with broad and potent antibody responses that were elicited in response to infection by the most prevalent subtypes of HIV-1. We believe the approach proposed here, which focuses on a unique longitudinal cohort of HIV-1 infected subjects has the potential for high payoff, as it could ultimately provide insights into the types of antibodies that are needed for effective protection by vaccination.